Re: [PATCH] io_uring: don't touch 'ctx' after installing file descriptor

[Jens Axboe <axboe@kernel.dk>]

On 7/31/20 1:58 AM, Stefano Garzarella wrote:
> On Thu, Jul 30, 2020 at 01:47:52PM -0600, Jens Axboe wrote:
>> As soon as we install the file descriptor, we have to assume that it
>> can get arbitrarily closed. We currently account memory (and note that
>> we did) after installing the ring fd, which means that it could be a
>> potential use-after-free condition if the fd is closed right after
>> being installed, but before we fiddle with the ctx.
>>
>> In fact, syzbot reported this exact scenario:
>>
>> BUG: KASAN: use-after-free in io_account_mem fs/io_uring.c:7397 [inline]
>> BUG: KASAN: use-after-free in io_uring_create fs/io_uring.c:8369 [inline]
>> BUG: KASAN: use-after-free in io_uring_setup+0x2797/0x2910 fs/io_uring.c:8400
>> Read of size 1 at addr ffff888087a41044 by task syz-executor.5/18145
>>
>> CPU: 0 PID: 18145 Comm: syz-executor.5 Not tainted 5.8.0-rc7-next-20200729-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:77 [inline]
>>  dump_stack+0x18f/0x20d lib/dump_stack.c:118
>>  print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
>>  __kasan_report mm/kasan/report.c:513 [inline]
>>  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
>>  io_account_mem fs/io_uring.c:7397 [inline]
>>  io_uring_create fs/io_uring.c:8369 [inline]
>>  io_uring_setup+0x2797/0x2910 fs/io_uring.c:8400
>>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> RIP: 0033:0x45c429
>> Code: 8d b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 5b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f8f121d0c78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
>> RAX: ffffffffffffffda RBX: 0000000000008540 RCX: 000000000045c429
>> RDX: 0000000000000000 RSI: 0000000020000040 RDI: 0000000000000196
>> RBP: 000000000078bf38 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000078bf0c
>> R13: 00007fff86698cff R14: 00007f8f121d19c0 R15: 000000000078bf0c
>>
>> Move the accounting of the ring used locked memory before we get and
>> install the ring file descriptor.
> 
> Maybe we can add:
> Fixes: 309758254ea6 ("io_uring: report pinned memory usage")

For sure, I just checked and had sort of assumed this existed in earlier
versions too, but it is indeed a recent issue. I'll add the Fixes tag
and your Reviewed-by, thanks!

-- 
Jens Axboe